The present invention generally relates to a method for cleaning semiconductor wafers and more particularly, relates to a method for cleaning semiconductor wafers with ozone-containing solvent.
In the manufacture of semiconductor devices, a large quantity of deionized (DI) water is required to process silicon wafers. The consumption of DI water increases with the size of the wafers. For instance, the consumption at least doubles in the processing of 200 mm size wafers when compared to the consumption in the processing of 150 mm size wafers. DI water is most frequently used in tanks and scrubbers for the frequent cleaning and rinsing of wafers in process. It is desirable that the surface of a wafer be cleaned by DI water after any process has been conducted on the wafer, i.e., oxide deposition, nitride deposition, SOG deposition or any other deposition or etching process. Such wafer cleaning step is accomplished by equipment that are installed either in-line or in a batch-type process.
For instance, a cassette-to-cassette wafer scrubbing system is one of the most used production systems for wafer cleaning prior to either a photoresist coating, oxidation, diffusion, metalization or CVD process. A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high pressure spray of DI water with a retractable cleaning brush. A typical mechanical scrubbing process consists of rotating a brush near a wafer surface that is sprayed with a jet of high pressure DI water at a pressure between about 2,000 and about 3,000 psi. The brush does not actually contact the wafer surface, instead, an aquaplane is formed across the wafer surface which transfers momentum to the DI water. The movement of the DI water thus displaces and dislodges contaminating particles that have been deposited on the wafer surface. Contaminating particles are thus removed by a momentum transfer process. As a result, larger particles become more difficult to dislodge and remove from a wafer surface.
A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. In a typical wafer scrubbing equipment, production rates are generally between 60 to 120 wafers per hour depending on the program length. The spinning speed of the wafer is between 500 to 10,000 rpm while under a water pressure of up to 6,000 psi.
In more recently developed wafer scrubbing systems, in-line systems are used which provide high pressure DI water scrubbing only while eliminating the possibility of wafer contamination by overloaded brushes. The water pressure in these systems range between 3,000 to 6,000 psi which are ejected from a nozzle mounted on an oscillating head. The wafer is spun when the oscillating spray is directed onto the wafer surface. After the cleaning step, wafer is dried by a pure nitrogen gas purge to promote rapid drying. After the scrubbing operation, wafers can be loaded into an in-line dehydration baking system for thorough drying. Batch-type systems are also used with DI water for cleaning, rinsing and drying prior to many IC processes. The systems can be programmed wherein wafers are loaded in cassettes before each cycle. One disadvantage of the batch system is their inability to be integrated into part of an automated wafer processing line.
In the conventional DI water cleaning systems, the basic requirements for the DI water cleaning system are that it provides a continuous supply of ultra-clean water with very low ionic content. It is believed that ionic contaminants in water, such as sodium, iron or copper when deposited onto a wafer surface can cause device degradation or failure. It is therefore desirable to eliminate all such ionic content from a DI water supply prior to using the water for cleaning wafers. A conventional method of measuring the ionic content in DI water is by monitoring the water resistivity. A water resistivity of 18xc3x97106 Ohm-cm or higher indicates a low ionic content in the DI water. In a conventional water purifying system, several sections which include charcoal filters, electrodialysis units and a number of resin units to demineralize the water are used for purifying the water.
Deionized water is frequently used in a wet bench process after a metal etching process has been conducted on a semiconductor wafer. When residual etchant chemical must be removed, deionized water rinse is used in a wet bench process for semiconductor wafer processing to perform two major functions of a quick dump rinse (QDR) and a cascade overflow rinse. Conventionally, the two functions are carried out in separate tanks in order to produce the desirable result. One of the major processing issues presented by the conventional dual-tank process is the particle re-deposition problem during a withdrawal step when cassettes are transported from a quick dump rinse tank to a cascade overflow tank. A second major issue is the large floor space required for accommodating the two tanks.
A conventional wet bench wafer cleaning process is shown in FIG. 1. The wet bench wafer cleaning process 10 for cleaning wafer 12 is carried out in six separate cleaning and rinsing tanks sequentially of a HF cleaning tank 14, a first quick dump rinse (QDR) tank 16, a SC-1 cleaning tank 18, a second quick dump rinse tank 20, a SC-2 cleaning tank 22 and a third quick dump rinse tank 24. The first HF cleaning tank is used to hold a diluted HF solution, for instance, at a concentration of 0.5% HF in H2O for removing a thin native oxide layer from the wafer surface. After the diluted HF cleaning process, the wafer 12 is rinsed in a first quick dump rinse tank 16 with deionized water. Wafer 12 is then cleaned in a second cleaning tank filled with SC-1 cleaning solution, i.e. a mixture of NH4OH, H2O2 and DI water at a ratio of 1:1:5. The SC-1 cleaning solution is used at a temperature between 70xcx9c80xc2x0 C. for a suitable time period. The wafer 12 is then rinsed again in a second quick dump rinse tank 20 that is filled with DI water. In the final stage of cleaning, the wafer 12 is cleaned in tank 22 filled with a cleaning solution of SC-2 which is a mixture of HCl, H2O2 and DI water at a ratio of 1:1:6. The wafer 12 is then rinsed in a third quick dump rinse tank 24 with DI water.
The wet bench wafer cleaning process 10 shown in FIG. 1 is conventionally used for pre-diffusion clean, pre-gate oxidation clean, pre-CVD clean, etc. For instance, in the ULSI fabrication of integrated devices, the conventional wet bench wafer cleaning process 10 can be advantageously used for wafer surface cleaning before a coating process in a CVD chamber or an oxidation process in a furnace.
Recently, ozone-injected ultrapure water has been used in the semiconductor fabrication industry to remove organic contaminants on a silicon surface that are caused by organic vapors in the ambient, or the residue of photoresist material. It has been discovered that when ozone is dissolved in ultrapure water, it decomposes and becomes a strong oxidizing agent that decomposes organic impurities. The ozone-injected ultrapure water cleaning process has the advantages of lower operating temperature, simplicity in operation, and reduced chemical consumption. One drawback of the ozone-injected ultrapure water cleaning process is that it causes the growth of native oxide on the silicon wafer surface due to the oxidizing effect of ozone. The growth of the oxide thickness increases as the immersion time in the ozone-injected water increases. The oxide thickness further increases with the concentration of ozone. Hence, even though the ozone-injected ultrapure water is an effective cleaning method for removing organic residue from a silicon surface, it cannot be integrated into a total wafer wet cleaning process unless and until the oxide growth issue can be resolved.
It is therefore an object of the present invention to provide a method for cleaning a semiconductor wafer by wet bench that does not have the drawbacks or shortcomings of the conventional wet bench process.
It is another object of the present invention to provide a method for cleaning a semiconductor wafer that incorporates an ozone cleaning step without incurring the oxide growth problem.
It is a further object of the present invention to provide a method for cleaning a silicon wafer by a wet bench method incorporating ozone dissolved in deionized water (DI water).
It is another further object of the present invention to provide a method for cleaning a silicon surface by a RCA cleaning cycle incorporating rinsing steps with ozone dissolved in deionized water.
It is still another object of the present invention to provide a method for cleaning a silicon surface in a RCA cleaning cycle, rinsed by DI water containing ozone, and then cleaned by diluted HF for removing any oxide layer formed on the silicon surface.
It is yet another object of the present invention to provide a method for cleaning a silicon surface in a multi-bath process that includes SC-1 and SC-2 cleaning steps followed by a rinsing step utilizing ozone dissolved in DI water.
It is still another further object of the present invention to provide a method for cleaning a silicon surface in a multi-bath process in which the surface is first cleaned by a cleaning solution containing NH4OH, rinsed by DI water containing O3, cleaned by a second cleaning solution containing HCl, and rinsed by DI water containing O3.
In accordance with the present invention, a method for cleaning a silicon surface in a multi-bath process incorporating the step of rinsing by DI water containing ozone is disclosed.
In a preferred embodiment, a method for cleaning a semiconductor wafer can be carried out by the operating steps of cleaning the wafer in a first tank by a first solution which includes a base or an acid; and cleaning the wafer in a second tank by a second solution that includes DI water and between about 1 ppm and about 20 ppm ozone.
The method for cleaning a semiconductor wafer may further include a rinsing step after the cleaning step, or a rinsing step by DI water after the cleaning step. The second solution may include preferably between about 3 ppm and about 10 ppm ozone. The first solution may include ammonium hydroxide or hydrochloric acid. The first solution may include NH4OH, H2O2 and H2O. The first solution may further include HCl, H2O2 and H2O. The second solution may include DI water and ozone, the second solution may further include diluted HF and ozone. The method may further include the step of cleaning the wafer by a second solution in a quick-dump-rinse operation, or in a cascade overflow rinse operation.
The present invention is further directed to a method for cleaning a semiconductor wafer in a multi-bath process which can be carried out by the operating steps of exposing the semiconductor wafer to a first cleaning solution that includes NH4OH; rinsing the semiconductor wafer by a first rinsing solution including O3; exposing the semiconductor wafer to a second cleaning solution including HCl; rinsing the semiconductor wafer by a second rinsing solution including O3; and drying the semiconductor wafer.
The method for cleaning a semiconductor wafer in a multi-bath process may further include the steps of, after the rinsing by the second rinsing solution and prior to the drying step, exposing the semiconductor wafer to a third cleaning solution including diluted HF; and rinsing the semiconductor wafer by a third rinsing solution of DI water. The method may further include the step of exposing the semiconductor wafer to a first cleaning solution that includes NH4OH, H2O2 and DI water, or the step of exposing the semiconductor wafer to a second cleaning solution including HCl, H2O2 and DI water. The method may further include the step of rinsing the semiconductor wafer by the first and the second rinsing solution that includes between about 1 ppm and about 20 ppm ozone, or preferably includes between about 3 ppm and about 10 ppm ozone. The method may further include the step of exposing the semiconductor wafer to the third cleaning solution that includes less than 1 vol. % HF in DI water, or the step of drying the semiconductor wafer by a spin drying technique, or the step of drying the semiconductor wafer by an IPA vapor drying technique.